Basically, make it impossible to use more than a certain number of heirloom points in one's equipment.
For example, if I use a +3 deli cap and a +3 KAS, I'm using 6 heirloom points in total.
Depending on the limit (between 5 and 8 seems reasonable to me), this would introduce a new element in build-design. Players would need to make an interesting choice on how to use those limited heirloom points. There would be a real tradeoff between using one heirloomed piece of equipment instead of another. Furthermore it would level the playing field by limiting the heirloom advantage of long term players, and shortening the "grind to maximum effectiveness". This would be a small nerf to ranged, throwers, cav and shielders because those classes use more heirloom points. This would also open up the possibility of making heirloom bonuses bigger again. The trend has always been to nerf heirlooms but if there's a limit to how many heirloom points a long term player can have more than a starting player, then we can think about making heirlooms more "unique" and powerful like they used to be a long time ago.